by Dmitry Zaks, Agence France-Presse

KIEV - The United States accused Russia of sending tanks and rocket launchers to pro-Moscow rebels in Ukraine as Kiev's resurgent forces hoisted the national flag over a strategic rebel-held port amid fears of possible Russian gas cuts.

Moscow said Ukrainian tanks had crossed the border into its territory before being intercepted, a day after Ukraine alleged that three tanks had crossed from Russia into its territory, underscoring the growing tensions between Kiev and Moscow.

Ukraine's President Petro Poroshenko told his Russian counterpart Vladimir Putin that the tanks' crossing into the former Soviet republic's separatist east was "unacceptable".

Washington backed Kiev's claims, saying separatists in eastern Ukraine had also obtained multiple rocket launchers from Russia among other heavy weapons and military equipment.

The United States has "information that Russia has accumulated tanks of a type no longer used by Russian forces at a deployment site in southwest Russia, and some of these tanks recently departed," State Department deputy spokeswoman Marie Harf said Friday.

"We are confident that these tanks came from Russia," she added, noting that no Ukrainian tank units have been operating in the area.

Earlier, she told reporters that a convoy of three T-64 tanks, several BM-21 or Grad multiple rocket launchers and other military vehicles had "crossed from Russia into Ukraine".

Internet videos had shown the same types of tanks moving through multiple cities in eastern Ukraine, including Snizhne, Torez and Makiyivka, Harf said.

The same types of rocket launcher was seen travelling through Lugansk.

NATO chief Anders Fogh Rasmussen warned that reports of pro-Russian groups in eastern Ukraine acquiring heavy weapons from Russia, including tanks, would mark a "serious escalation" of the crisis if confirmed.

Poroshenko celebrated the "heroism" of soldiers hoisting the national flag over the strategic rebel-held port of Mariupol.

He proclaimed the industrial Sea of Azov city the new temporary capital of Donetsk -- an eastern rustbelt region overrun by pro-Russian gunmen for the past two months.

Mariupol has wavered between rebel and Kiev control for weeks and was also the scene of pitched battles on May 9 that killed more than a dozen people.

- Heavy rebel losses -

The bustling port of half a million people provides access to the main highway linking other regions with Russia and is the main export channel for coal and industrial products fundamental to Ukraine's economic growth.

Ukrainian Interior Minister Arsen Avakov said that federal forces led by the part-volunteer National Guard had inflicted "heavy losses" on the rebels while seeing only four soldiers suffer light wounds.

But the prosecutor's office also confirmed that three soldiers died late Thursday when they were ambushed by the militia in the Donetsk region town of Stepanivka.

His deputy told reporters that 30 pro-Russian gunmen had been captured in a coordinated push involving special forces that saw rebels driven from the city's seat of power they had reoccupied about a month ago.

The region's main administration building in Donetsk remains under rebel control.

Poroshenko rose to power in a snap May 25 ballot called after the February ouster of a Russian-backed leader by vowing to move Ukraine closer to the West and end fighting that has claimed 270 lives.

But the battles have since only intensified and his calls for dialogue with more moderate separatist leaders have been mostly ignored.

- Monday morning gas cut -

Poroshenko's troubles have been compounded by the threat of Ukraine being cut off from economically-vital Russian gas shipments as early as Monday morning because of a bitter price dispute.

Prime Minister Arseniy Yatsenyuk told regional administrations and the heads of the state energy company to prepare for the consequences of a possible gas cut by implementing a plan for reduced energy use.

Moscow said Friday Ukrainian tanks had crossed the border into its territory before being intercepted by the border patrol.

The allegation came a day after Ukraine alleged tanks had crossed from Russia into its territory, and underscored the growing tension between Kiev and Moscow, fuelled in part by the lack of any progress in the high-stakes "gas war".

Ukraine receives half its gas supplies from Russia and transports 15 percent of the fuel consumed in Europe.

The head of Ukraine's state energy firm said Kiev was ready to make a $1.95-billion (1.45-billion-euro) payment demanded by Moscow by Monday if Russia settled on a price of $326 per 1,000 cubic metres of gas.

A spokeswoman for Russia's energy minister also refused to confirm EU Energy Commissioner Guenther Oettinger's earlier suggestion that new negotiations might be held on Saturday.

Analysts said the talks' slow progress reflected the complete loss of trust between the neighbours and the high stakes involved in the broader East-West battle for the future of the ex-Soviet state.

"We see gas talks as the derivative of the wider dispute between Russia and Ukraine," New York's Morgan Stanley investment bank said in a research note.

European Commission chief Jose Manuel Barroso meanwhile told the Russian leader he was willing to include Moscow in discussions with Kiev weeks before the EU is due to sign an association accord and free trade pact with the former Soviet state.